1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to improved line interface circuitry providing more efficient component utilization at a central office of a telecommunications system.
2. Discussion of the Related Art
In recent years telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately 300 hertz to approximately 3.4 kilohertz.
New, high-speed data communications provided over digital subscriber lines, such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), etc. (more broadly denoted as xDSL) provide for high speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for xDSL transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and xDSL signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.
A POTS splitter is typically provided at each end of a transmission system communicating both POTS and xDSL information. As is known, a POTS splitter circuit consists of a high-pass filter and a low-pass filter, which are used to separate the two signals (the POTS signal from the xDSL signal). Normally, the high-pass filter is built into the xDSL transceiver, whereas the low-pass filter (commonly referred to as a POTS filter) normally provided as a separate unit. In operation, the POTS filter operates to filter the high frequency xDSL signals in order to protect the POTS (e.g., telephone) circuitry. At the same time the POTS filter provides filtering and protection for higher frequency noise signals that are often associated with a ring signal, or the switching circuitry of a central office.
A "line-card", containing line interface circuitry, is provided at the central office. The line interface circuitry provides the interconnections among xDSL circuitry, POTS or PSTN voice circuitry, off-hook (or tip/ring) detection circuitry, ring generator circuitry, and the local loop. As is known, the line interface circuitry includes a POTS filter that is interposed between the various POTS circuits and the xDSL circuit. This filter protects the POTS circuitry from the high frequency signals of the xDSL transmission circuitry. It also serves to minimize noise transmissions across the local loop from the central office to the customer premises.
As is known, upon entering a line card offering both POTS and xDSL services, the local loop splits into at least two distinct channels: a POTS channel and an xDSL channel. The POTS channel is buffered by a low-pass POTS filter from the higher frequency xDSL signals. Similarly, the xDSL channel is typically buffered by either a band-pass filter or a high-pass filter defined by a pass band to filter signals in the lower frequency POTS band. Beyond these filters, the two channels (i.e., the POTS channel and the xDSL channel) have similar features. For example, each channel is further split into a transmit side and a receive side. The transmit side of each channel typically includes a digital to analog converter, while the receive side includes a companion analog to digital converter. Using these converters, outgoing signals for transmission over the local loop are converted from digital to analog format. In the same way, incoming signals received from the local loop are converted from analog to digital format, so that they are placed in a form suitable for passing through a digital switch at the central office, or alternatively for processing by a digital signal processor (DSP).
It is recognized that the circuitry provided on many central office line cards is unduly duplicative. Accordingly, it is desired to design an improved line card circuit having reduced component count. Advantageously, such an improved circuit design would necessarily result in lower implementation costs and improved reliability.